Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-MRAM). STT-MRAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
FIG. 1 depicts a conventional dual magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-MRAM. The conventional dual MTJ 10 typically resides on a bottom contact (not shown), uses conventional seed layer(s) (not shown) and may include conventional antiferromagnetic (AFM) layers (not shown) to fix, or pin, the magnetic moment of the pinned layers. The conventional magnetic junction includes a first conventional pinned layer 12 having magnetic moment 13, a conventional tunneling barrier layer 14, a conventional free layer 16 having changeable magnetic moment 17, a second conventional tunneling barrier layer 18 and a second conventional pinned layer 20 having a magnetic moment 21.
The conventional pinned layers 12 and 20 and the conventional free layer 16 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction. For the perpendicular-to-plane pinning direction desired for the conventional pinned layer 12 and 20, the conventional pinned layers 12 and 20 each has a substantially stable magnetic moment 13 and 21, respective. Although depicted as a simple (single) layers, the conventional magnetic layers 12, 16 and 20 may include multiple layers. For example, the conventional magnetic layers 12, 16 and/or 20 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic.
To switch the magnetization 17 of the conventional free layer 16, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven through the free layer 16, the free layer may be switched to be parallel to the magnetic moment 13 or antiparallel to the magnetic moment 13. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
To enhance the spin-transfer based switching, the conventional pinned layers 12 and 20 are desired to be in a “dual state”, as shown. Stated differently, the magnetic layers of the conventional pinned layers 12 and 20 that are closest to the conventional free layer 16 are desired to have their magnetic moments in opposite direction. Typically, this dual state is achieved by ensuring that the coercivities of the pinned layers 12 and 20 are significantly different. Thus, a high magnetic field may be applied along the easy axis to align the magnetic moments 13 and 21. A lower magnetic field may then be applied in the opposite direction to switch the magnetic moment of one of the pinned layers 12 or 20, while leaving the magnetic moment of the other pinned layer 20 or 12 unchanged.
Although the conventional dual MTJ 10 may be written using spin transfer and used in an STT-MRAM, there are drawbacks. For example, the write error rates (WER) may be higher than desired for memories having an acceptable Ic0 and pulse width. The write error rate (WER) is the probability that a cell (i.e. the magnetic moment 17 of free layer 16 of the conventional dual MTJ 10) is not switched when subjected to a current that is at least equal to the typical switching current. For example, the conventional dual MTJ 10 may be subject to back hopping. The write current driven through the conventional MTJ 10 may also destabilize the magnetic moment(s) of the pinned layers 12 and/or 20. The magnetic moment 17 of the conventional free layer 16 that has been switched using spin transfer torque may switch back (or hop back) to its initial state. This back hopping and the attendant increase in error rate is undesirable. Further, the dual state for the magnetic moments 13 and 21 of the pinned layers 12 and 20 depicted in FIG. 1 may be difficult to achieve in a magnetic memory including an array of conventional dual MTJs 10. In particular, variations between individual conventional dual MTJs 10 in the array may preclude setting most or all of the array using two fields as discussed above. This may render some magnetic junctions unusable.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.